WHAT, EXACTLY, is a wine bar? The question is intended seriously. There has been a good deal of talk about wine bars of late. They have become popular in London, in San Francisco, in New York and now they have come to Washington.

Wine, as everyone knows, is part of today's scene. It's trendy, an "in" beverage, or whatever more refined term a sociologist might choose. And Washington, as everyone knows, is a wine town. But is it going to be a wine-bar town?

The first local rendition of a wine bar, Le Grenier au Vin, above La Chaumiere restaurant in Georgetown, faded away and soon became an upstairs dining room with an extensive wine list.The second and the third attempts, the Carlton Wine Bar and Chez Maria, are newly alive and full of hope. But their future is as uncertain as is the purpose of the wine bar itself.

Are wine bars for wine buffs, for singles, for abstainers from the hard stuff? Should they serve small portions of rare wines, large bumpers of budget wines, sponsor parties, act as information exchanges, swing or be sedate?

You won't get a single answer anywhere, not even in this town where the choice is between two, not among several. No two enterprises could be more diametrically opposed in style and concept than the sleek, private-club-like Carlton Wine Bar in the Sheraton Carlton and the homey, hand-built basement wine bar in Chez Maria, a very good Vietnamese restaurant on M Street near Key Bridge that last housed the Gate Soup Kitchen. For all the apparent differences, however, they share a purity of purpose unmatched by the wine bars I've encountered elsewhere. They have been created in the (perhaps naive) belief that a significant portion of the local public is excited by wine, hungry for the opportunity to sample and study it in friendly surroundings and eager to make wine tasting a social sport.

"I want to do something serious," said Roger Grison, the middle-aged French owner-host of Chez Maria."i want my guests to learn about wine and I want to learn with them. There are many things I do not know, especially about California wines."

Grison is an engineer. Chez Maria is his first restaurant venture. The cooking is in the hands of his wife, whom he met and married in Vietnam in the late 1960s. "The wine tasting bar is more fun than business," he said. "I told Maria she would run the restaurant and I would run my small export/import company and go fishing. But I like to work and it is pleasant to meet the guests, so I work almost every day now." The restaurant is open for lunch, but the wine bar does not open until 6:30 p.m. each day.It closes, according to Grison, "when no guests are left."

Grison ambles about, introducing himself and his wines in English that is filled with echoes of a Maurice Chevalier. He has been receiving a hand from Russ Everett, who works across the street at Eagle Wines and Liquors during the day and currently is prospecting in France.

Each week at Chez Maria there is a "theme," four wines that can be tasted together systematically. Last week, for instance, the four were "beaujolais" (gamay) from France and California. Other themes since the wine bar opened in mid-November have included California chardonnay, Chateau St. Jean rieslings, champagne and sparkling wines. In addition, six or eight other wines are available "on special" by the glass or bottle and Grison will provide evaluation sheets for those who wish them. There is a longer wine list as well. Prices are modest. Most evenings will be pate, cheese and bread available for those who want food with their wine, plus shrimp chips, an Oriental touch that works amazingly well as a foil for wines with residual sugar. Full meals must be taken upstairs, however.

There is a real bar in the wine bar, and a scattering of tables. Capacity is about 50, but Grison admits he has not been overwhelmed with customers. Is he discouraged? "Absolutely not! I hope in one, two or three months to have a good business," he says.

Those who have come are "mature" and I have included parties of women and men as well as couples. They divide between sippers who just try a single wine and tasters. Grison's enthusiasm is clearly for the tasters. "What I like," he said, "is the possibility to compare wine made from the same grape. For example, this week we tasted the Beaujolasis and I tried the one from California, from Sebastiani. It was absolutely outstanding, like what I remember tasting in Lyon 20 years ago."

They've offered the new Beajolais at the Carlton, too. The week before Christmas Michael Lavenson, who commands the wine bar, featured two Beaujolais nouveaux from France, the Sebastiani and Boordy Vineyard's tirage en primeur from Maryland. "From the start (in November)," he said, "we'ved taken the attitude that American wines are what is making wine drinking so popular and exciting. Sheraton is convinced that wine bars are a trend, not a fad, and we wanted to try to create a new approach to them."

The concept, as Lavenson capsulized it, is that of a British drawing room with an American-oriented list of wines, shaffed by knowledge young people with an unpretendious approach to the subject. As cellar master for the hotel, Lavenson has compiled a list of about 100 wines, of which only about 30 percent are imported. Weekly specials, sold by the glass or by the bottle, appear on a special list. The same week the beaujolais were featured, for example, there also were seven white wines from Chateau St. Jean. Prices ranged from $1.75 per glass for the American nouveau wines to $4 for a glass of St. Jean's blanc du blanc sparkling wine.

The Carlton, which has been dramatically brought back to life by manager Rose Narva, was chosen for this experiment, according to Lavenson, because of Washington's reputation as a wine and entertaining center and because the Carlton has been gaining a reputation as a trend-setter among local hotels.

The former bar of the Federal City Club has been redecorated into a room that is formal, handsome and restrained. There are outsized chairs, couches and a library of wine books. Tables may moved about, but capacity remains between 35 and 40. It is open only from 4 to 8 p.m., though a later closing hour is being contemplated. Some light food, such as pate and cheese, is usually available. In keeping with the club-like mood, ports, sherries and brandies are available as well as table wines. There is no "house" wine. Instead Lavenson and two assistants, David Key and Robert Whale, try to match guests' tastes with various styles of wines. About half the sales at this point are by bottle or half-bottle. All three know their business and act relaxed and unstuffy. Their manner helps to offset the potential intimidation factor of such a grand setting. About half the sales now are by bottle or half-bottle.

Lavenson has worked hard to establish contacts with the local wine trade, wine groups and individual consumers. Several wine functions have been held at the Carlton and more are planned. Guests who stay at the hotel are invited to the wine bar for a complementary glass of wine. Lavenson is pleased at a return customer level he puts at 30 percent, and thinks the wine bar can become a positive factor in attracting visitors to choose the Carlton over other local hotels.

Chez Maria has now prepared a schedule that runs through April. It, too, plans to stick out the winter months. In addition, there is a growing number of restaurants that --while stopping short of declaring themselves wine bars -- have developed an appealing selection of wines by the glass. Among those are Coleman's, at Pennsylvania and 20th St. NW, Beaujolais in the One Washington Circle Hotel, the American Cafe on Massachusetts Avenue SE and Wisconsin Avenue in Georgetown, Nathan's (also in Georgetown) and the Garden Terrace of the new Four Seasons Hotel. Two other gathering places for local oenophiles are the bar of the Prime Rib, presided over by Jim Ross, and the Old Europe, which has an imposing and attractively priced list of German wines.

What are wine bars? The jury still is out. In addition to the two now operating there is talk that Steven Spurrier, the English merchant who sells wine in Paris, may sponsor another and rumors that a national chain of wine bars may soon emerge with Washington targeted for an early franchise.

Why wine bars? Roger Grison supplied the answer. "In 1963," he said, "When I first visited the U.S., nobody drank wine. Today if I fly from New York City to Washington they offer me a glass of wine. It's unbelieveable that things have changed so much."